The present invention relates to a frequency converter for use in a radio communication, and a radio receiver using same. More specifically, the invention relates to a frequency converter which can be driven by a small local oscillation signal, and a radio receiver which can prevent the self-mixing in a direct-conversion receiver and so forth by means of the frequency converter.
An example of conventional frequency converters is illustrated in FIG. 1. A signal inputted from a radio-frequency (RF) terminal 1 is inputted to a base of a first transistor Trl via a high-frequency input circuit (not shown). Local oscillation signals inputted from first and second local oscillation (Lo, *Lo) terminals 2 and 3 are also inputted to bases of second and third transistors Tr2 and Tr3 forming a differential pair 4, respectively. Furthermore, the signal *Lo is a inverted signal of the signal Lo. As described in "Analysis and Design of Analog Integrated Circuits" written by P. R.Gray and R. G.Meyer, the differential pair 4 of transistors is operative to distribute the collector current of the first transistor Tr1 to the second and third transistors Tr2 and Tr3 on the basis of the potential difference between the bases of the transistors Tr2 and Tr3. The collector current is converted into a voltage output by means of a load circuit 5 to be outputted as an output Vout. The load circuit 5 comprises fourth and fifth transistors Tr4 and Tr5.
The frequency converter using the differential pair of transistors causes the switching of the first and second transistors Tr2 and Tr3 forming the differential pair by applying great voltage amplitudes to both of the base terminals thereof in order to decrease the fluctuation of conversion gain. In this case, the output can be expressed by the following formula (1). EQU Vout(t)=K.times.F(t).times.{Irt(t)+Iee} (1)
wherein Irf is a high-frequency signal current outputted from the collector of the transistor Tr1, Iee is a bias current flowing through the collector of the transistor Tr1, F(t) is a function which is alternately 1 and -1 at the same frequency as that of the local oscillation signal, and K is a constant determined by the load circuit.
F(t) contains frequency components integer times as large as the frequency of the local oscillation signal. A desired signal is the product of a fundamental-wave component sin(2.pi.ft) and Irf wherein f is a local oscillation frequency.
Assuming that "Ifr=A(t) sin(2.pi.frft)", there can be expressed by the following formula (2). EQU K/2A(t)[cos{2.pi.(frf-f)t}-cos{2.pi.(frf+f)t}] (2)
Using a low-pass filter, the signal having a carrier frequency of frf can be converted into a signal having a carrier frequency of frf-f by taking out a signal component of "A(t) cos{2.pi.(frf-f)t)" in the aforementioned formula.
As described in "Low-Power Radio-Frequency IC's for Portable Communication" written by Asad Abide (Processings of the IEEE, vol. 83, No. 4, April 1995), in a case where such a frequency converter circuit is used for a direct-conversion receiver, the local oscillation signal leaks out of a high-frequency signal input terminal to be reflected due to the impedance mismatch of a low-noise amplifier circuit, an antenna and so forth, to be superposed on the high-frequency input signal Irf to be inputted to the frequency converter. This signal is mixed with the original local oscillation signal to be a DC offset. In addition, if the reflection amount fluctuates in accordance with the environmental fluctuation of the antenna, the signal may be a low-frequency noise due to the fluctuation of the offset.
In the direct-conversion receiver, since a received signal is converted into an approximately direct-current baseband signal, it is impossible to separate a DC offset and/or a low-frequency noise from a desired signal by means of a filter, so that the deterioration of communication quality is caused. This problem is called self-mixing of a local oscillation signal. As means for solving this problem, Ito and Kawakami has proposed the use of an even higher-harmonic using an anti-parallel diode pair, in "Even Order Mixing Products of Even Harmonic Mixer for Direct Converter (1995 General Conference of Institute of Electronics Information and Communication Engineers, Lecture Number C-87).
The voltage-current characteristic of a diode can be expressed by "I=Is{exp(.alpha.V)-1}" wherein Is and .alpha. are constants determined by the type of element. When the Taylor expansion of "exp(x)" in the aforementioned formula is performed, the following formula is derived. ##EQU1## wherein X contains terms of even and odd degrees. In this case, the anti-parallel diode pair comprises a pair of diodes of the same characteristic which are connected to each other in the opposite directions so that the terms of even degrees cancel out. Therefore, the anti-parallel diode pair has an odd functional characteristic as a whole. If the frequency of a local oscillation signal is set to be half of the frequency of a high-frequency signal, a tertiary distortion component, wherein the secondary distortions of the high-frequency signal and the local oscillation signal are mixed with each other, is outputted as a desired baseband signal. On the other hand, although the local oscillation signals reflected in the antenna, the low-noise amplifier circuit and so forth are mixed with the original local oscillation signal, the self-mixing of the local oscillation signal is decreased since only the secondary distortion components (square components) are converted into approximately direct currents.
However, in a case where the anti-parallel diode pair is used, it is required to input a great local oscillation signal in order to decrease the conversion loss. For example, as explained by Takahashi and Kuwatsuru in "An FSK Direct conversion Receiver Improving odd order of intermodulation" (1993 Fall Conference of Institute of Electronics Information and Communication Engineers, Lecture Number B-329), although a frequency converter circuit of a differential amplifier circuit type is operated by inputting a local oscillation signal of 90 dB .mu.V and 90 mvp-p, it is required to input a local oscillation signal of at least 0 dBm and about 630 mvp-p in a case where an anti-parallel diode pair is used, as described in the measured results by Ito et. al.
This describes that an even higher-harmonic mixer based on an anti-parallel diode pair utilizes a conducting state and a non-conducting state of a diode. That is, the frequency conversion is realized by the transitions of the following three states:
(1) a state wherein one of diodes is in a conducting state, PA1 (2) a state wherein both of diodes are in a non-conducting state, and PA1 (3) a state wherein the other diode is in a conducting state. PA1 (1) a state wherein an input signal exceeds a positive limitation amplitude, PA1 (2) a state wherein the input signal is between a negative limitation amplitude and the positive limitation amplitude, and PA1 (3) a state wherein the input signal exceeds the negative limitation amplitude.
Thus, it is expected that if a great signal amplitude is outputted as a local oscillation signal, a receiver produces a great, undesired radiation, so that it is apprehended that the great, undesired radiation obstructs other radio communications. In addition, although it is desired to decrease the consumed power in order to drive a portable telephone and/or a portable radio terminal using cells, it is required to increase the consumed current of a local oscillator or a local oscillation signal amplifier circuit in order to obtain a signal of greater than 0.6 V at a high frequency.